Monochromator



Feb. 23, 1954 K. G. MAcLl-:ISH

MoNocHRoMAToR Filed Deo. 14, 1950 3 Sheets-Sheet 1 FIG.3.

IGNNET H G .MACLEISH hwentor l BH Cttornegs Feb. 23, 1954 K. G. MAcLElsH2,669,899 MoNocHRoMAToR Filed Dec. 14, 1950 3 Sheets-Sheet 2 WA vfl.ENGTH KENNETH G MCIEISH :inventor l ,QM f l B5 WMM,

Gttornegs Feb. 23, 1954 K. G. MAcLElsH 2,669,899

MoNocHRoMAToR Filed Dec. 14, -1950 3 Sheets-Sheet 5 ll/-JOI mmm G.MACLEISH :inventor Bg QM MN Gttomegs Patented Feb. 23, 1954 UNITEDSTATES PATENT GFFICE 2,669,899 MONOCHROMATOR Kenneth G. Macleish,Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., acorporation of New `lersey Application December 14, 1950, Serial No.200,819

13 Claims.

This invention relates to monochromators which are optical instrumentsof the spectroscope type for producing colored light of adjustable colorand usually of adjustable spectral band Width. A spectrometer may beconsidered a form of monochromator in which an eyepiece and nducial markreplace the exit slit of the system.

The objects of the invention are to provide 'a monochromator which iscompact and simple in construction, convenient and accurate in itsoperation, and of rugged and stable design so as to remain accurateunder continued usage.

Prior monochromators have taken many forms. The more common forms aredescribed in The Principles of Optics by Hardy and Perrin. The presentinvention is particularly applicable to the Fery monochromator describedon pages 556- 7 of the Hardy and Perrin text and also in U, Patent1,007,346, Fery. In monochromators and spectrometers there is a directsimilarity between the Fery spectrograph' or monochromator system andthe Rowland grating system described on pages 563-565 of this same text.In both cases there is a spectrum focus circle whose diameter is equalto the radius of curvature of the front surface of the Fery prism in onecase and the surface of the concave gratingin the other. The center orcurvature of the refracting or refleeting surface falls on this focuscircle` In the case of the prism the center of curvature of the rearsurface, which is the reflecting one, is also on the focus circle. Thepresent invention includes a number of features which are applicable toeither system, that is, to any monochromator 'of the type having areecting spectral dispersing element. Concave gratings have certaindisadvantages such as overlapping orders of spectra as compared withspherical reflecting prisms. Therefore the present invention isdescribed particularly with reference to the prism system, but it is tobe understood that the principles thereof are applicable to concavegratings.

One feature of monochromators of this general type is the slightdistortion or the lines of the spectrum. The normal procedure toovercome this distortion or to compensate for it is to have either theentrance slightly. Because of this slight curvature, one cannot statethat both slits are strictly straight, b'ut the curvature is very slightand with this possible refinement in mind, the slits can be described asbeing substantially perpendicular to the plane of dispersion of thereflecting element. Prior instruments did not, however, correct forastigmatism introduced by spherical refleeting dispersing elements. Oneof the objects and advantagesxof the present invention is the slit orthe exit slit curved l reduction or elimination of astigmatismintroduced in this Way. with both distortion and astigmatism minimized,an extremely eicient instrument results.

Thus the present invention may be considered primarily as an improvementin Fery monochromators, that is in moncchromators which have entranceand exit slits and spherical reilecting spectral dispersing elements,preferably prisms.

According to the invention a monochromator of this general type is madeup with fixed entrance and exit slits and a fixed prism, or other simpleor complex unit for dispersing and focusing light. These three elementsare rigidly attached to a support. That is, they are rigidly attached tothe support during operation of the instrument but are in generaladjustable during manufacture to insure proper calibration of theinstrument. Also the slits are usually of adjustable width and are fixedrelative to the optical system by having one jaw of each slit rigidlyattached to the support and the other one movable. Such slits are saidto be fixed since the change in slit width is very small compared to thedistance across the visible spectrum. In the present invention spectralselection is provided by means of a plane reflector associated with one`of the slits and movable so as to move a virtual image of that slitalong the focus circle of the monochromator.

The advantages of having the slits and prism all fixed relative to oneanother are simplicity of construction, ru-ggedness and stability ofoperation and the possibility of using the simplified slit constructiondescribed below. This latter possibility depends on the number cireflections in the optical system between the focusing means and the twoslits; between one slit and the focusing means, there should be one morereflection than between the other slit and the focusing means. Thisgeneral feature of iixed slits and a iixed prism also ties in with thesimplicity of 'the device for correcting astigmatism, namely thecylindrical mirror of constant curvature to be discussed below.

In most commercial uses of monochromators, extreme accuracy isnotnecessary and the present invention lends itself particularly well tothe design of an inexpensive but completely adequate commercialinstrument.

The mounting of the movable plane reflector may take any form whichcauses the virtual image of the associated slit to move approximatelyalong the spectrum focus circle of the monochromator. One preferredmounting which provides simplicity, ruggedness, stability andconvenience of coupling to the slit width adjusting mechanism isconstructed as follows. Two metal springs or ribbons of equal length arearranged parallel to each other and rigidi" at` and which moves therigid member and the plane f -f reflector transversely and withoutrotation.

Another feature of a preferred embodiment 'of' the invention consists ofa cylindrical mirror optically between the slits and the dispersingfocusing element, with its cylindrical axis in the plane 2f? ofdispersion. This arrangement reduces the astigmatism introduced by theObliquity atl which the light is incident on the spherical reectingmember and, if the curvature of the mirror is ap-A propriately selected,the astigmatism isreduced to a minimum practically to zero throughoutthe spectrum. This cylindrical mirror which forms a virtual image of theprism or other dispersing member on the spectrum focus circle ofmonochromator also contributes to the compactness of the instrument.When this cylindrical mirror is used the prism can be much smaller inthe direction perpendicular to the plane of dispersion; this reducescost as well as bulkiness to a minimum.

A third special feature of preferred embodiments of the invention isrelated to the slit construction for providing adjustable band width. Itis preferable in monochromators to adjust both slit widthssimultaneously, and by substantially or effectively equal amounts. Thisresults vin an energy distribution inthe vspectral band from theinstrument which is substantially triangular in form being a maximumapproximately at the center oi the spectral band, and falling i oli` torero at both sides ofthe band. In prior vinstruments in which one of theslits is movable, it is not convenient to have simultaneous adjustmentof the slit widths. The same is true of -instruments which have theslits at seme distance from each other. lf the light isnot reflected' atall between the dispersing member andthe slits or is reected the samenumberof times in both 'the entrance beam and exit beam, the adjustmentof' the slits is less convenient because it is necesvsary to move thetwo adjacent jaws ofthe slits or the two outside jaws and to move themin opposite directions. The selection of which jaw' moves, depends onthe iocusing arrangementv since itis preferable to have the entranceslit focused precisely onthe exit slit, one edge of the entrance lslitbeing focused on one edge of the exit slit `for some particularwavelength of light. The

movable jaws should therefore move so as to stay conjugate to oneanother for some particular wavelength. According to a preferred'embodiment of the present invention these advantages are obtained byhaving one jaw of each slitrigidly attached to the monochromatcr supportand the other jaw of each slit rigidly attached to-a single memberpivoted at a distance from the slits so that slight rotation of thismember adjusts both slit widths simultaneously. The. position of thepivot is selected to maintain the movable jaws conjugate to one anotherand approximately to insure that the degree of adjustment of both slitsis effectively the same. This latter requirement is not too criticalsince it affects only the distribution of energy in the spectral bandemitted and this is already affected somewhat by the residualaberrations of the system.

Finally a feature of the preferred embodiment ofthe invention involves acoupling of the slit adjusting mechanism to the spectral adjustmentmechanism so that the spectral width of the emergent band remainsconstant for all settings of` the spectral selecting device. In thepreferred embodiment of the invention this feature of constant bandwidth holds only for one particular band width and, at the same time,the spectral calibration of the instrument is linear.

VAlternatively the constant band width feature can be operated at two'or more band widths and in this. case the spectral calibration isnonlinear.

Gther objects and advantages of the invention will be apparent from thefollowing description when read in connection with the accompanyingdrawings in which:

Fig. lis a perspective view ofv a monochromator according to thepreferred embodiment of the invention; y

Fig'. 2 isa rear elevation ofthe device. shown ln Fig. l with the coverremoved;

Fig. 3 is a perspective view of part ofthe optical system;

lig. 4 is similar to Fig..2 and includes the adjusting mechanisms of theinstrument;

Fig. 5' isa perspective view of the slit adjusting mechanism;

lig. 6 is a diagram to, illustrate they operation of the mechanism showninFig. 5;

Fig. 7 graphically illustrates the spectral output of themonochromator';

Fis'. 8 is a perspective view of the spectral adjusting mechanism;

Figs. 9 and l0 are diagrams to illustrate the opera-tion of themechanism shown in Fig'. 8;

Fig. 1l is al diagram to illustrate the principles of the opticalsystem.

ln Fig. l a monochromator according to the invention consists of ahousing ll on which is mounted a light source l2 in va separate housingI3.. The light from the source passes through the. system and is,dispersed toform` a spectrum, apartl of which is selected, byy turningknob l5, and this partA then emerges.` from a window. I6 in the housing.The wavelength of the` emerging light, specifically the wavelcngthat.the. center of .theband is indicated by indexV l? and scale, i8 whiehismounted on a disk I9. which is rotated by the knobl5.. The entrance andexit slits of the mcnochromator-are coupled to be adjustedsimultaneously; This adjustment is provided by turning a knob. 2l andthe width of both slits is read by an index 22 and a scale 2,3 carriedon a. dislr; 24 which is turned by, the knob lll,

Fory one setting othe. slit; width, for example that which correspondsto a spectral band half width of; 10,0 the lmet 21 may be rigidlyattached by screw 25 to the disk I9. Rotation of the knob l5 ('or knob2l) then changes thc .wavelength as indicated by the. scale $8 and theslit width as indicated by the scale 23: simultaneously so that the bandwidth in angstrom units remains. constant. 'lin this. instrument theWavelength scale i8 is linear and the constant bandwidth feature holdsonly for one particu'- lar band width.Y Both adjustments involve. rotaryVcams and if the cam rotated by the knob 2i is chosen so as to give-theconstant band width 5 'feature' for several different bandwidths andfixed and then the mirrorv4| -is adjusted during -then the cam operatedby the knob I5 is cormanufacture to correct for any slightmisalignrespondingly modied, the wavelength scale I8 ment of the prism.y i' no longer would be linear. The lamp l2 is a small The slitadjusting mechanism is shown in Figs. -371/2 watt 5 volt lamp ofstandard sound repro- 5 4 and 5. The jaws 33 and 35 are rigidly attachedduction type and receives its power from an ordito the support 50 byposts 55. The other jaws of nary 5 volt transformer (not shown) througheach slit namely 34 and 36 are coupled together Wires 26. and rigidlyattached by posts 56 to a pivoted The essential features of the opticalsystem member 51 which is pivoted at the point 58v some are shown inFig. 2. It should be noted that the lil distance from the slits.Rotation of the memplane of dispersion is vertical if this view isconber 5'! about the pivot 58 is provided through a sidered anelevation; in considering the optics post 5B by rotation of a lever arm60 which carhowever and particularly in comparison with ries a camfollower 6| spring urged by a spring other monochromators it iscustomary to refer t2 against a rotary cam 53 which is rotated to theplane of dispersion as horizontal. Light l5 through a shaft 84 by theknob 2| shown in from the lamp |2 passing through a window 30 Fig. l.f.- l in the housing Il is focused by condenser lens 3| Reference toFig. 6 indicates the geometry of on an entrance slit'32 having jaws 33and 3-4. this slit adjustment. The ideal situation or at Jaw 34 isadjustable,simultaneously with one of least one ideal situation wouldhave both slits the jaws (namely 36) oi the exit slit 3l of the in- 20(or more exactly one slit and the virtual image strument. A field lens38 is included for focusing of the other in reflector 40) lying flushwith the the condenser lens 3| on the objective of the focus circle forall settings of the slit width. .In

z-whole system which in this case is a concave resuch case, the fixedjaws would have their edges .from the slit 32 is reflected by a planereflector 4t ilecting prism 42 to be described below. Light eifectivelyon the circle and the edges of the movable jaws would move effectivelyalongthe to a cylindrically concave reflector 4| whose cycircle. Itwould also be ideal to have bothvjaws lindrical axis lies in the planeof the drawing. remain effectively equal in width to each other :Thiscylindrical mirror 4| reflects the iight to a for all settings of theadjustment but this is not Spherally Curved IeeCT/ilg prism l2 Whose tooimportant and the present system only ap'- v'front surface 43 refractsand dispersas the light '30 proximates this latter condition. The rstconspectrally and whose rear surface 4d is provided dition allows forsome modification too. The

fwith a metallic coating to reflect and focus the important point is tohave the moving jaws re- 'lght- The Ieected and dispersed lE-ht asindimain conjugate to each other,i.e. remain focused cated by the rays45 strikes the mirror si again on each other. Thus one jaw may move oilthe fand is brought to focus on the exit slit 3l. The 35 focus circle ifthe other movable one simultanerays shown by broken lines in Fig. 2 arefor one ously moves oii` the circle in the proper direction @articularWavelength If White light from the to compensate for the oil circlemovement of the slit 32 is reflected by the mirrors 40 and 4| to rstjaw. This is a more general ideal and is th'e the prism, the returninglight from the prism 42 one accomplished by the arrangement shown. lisfocused to form a spectrum along a locus 40 The jaws 36 and 34 movealong the broken lines 'which passes through the slit 3'?. Adjustmentshown in Fig. 6, the critical part of the jaw being Aof the. mirror 4tparallel to itself Amoves this the slit edge, e. g. edge 'l2 in the caseof jaw 34.

spectrum locus transversely across the slit 3l so For these motions bothto be on the focus circle that any desired Wavelength may be selected.the pivot point 53 would have to be on the oppo- As ShOWIl in Fg- 3 theentrance Slt 32 is actui5 site side of the slits and outside theinstrument .ally slightly curved; the curvature is greatly exas shown.However, pivot 58 as shown is such `aggerated in Fig. 3. The purpose ofthis'slight that the motions are along conjugate focal lines curvatureis well known in. monochromators with respect to the image formingsystem which lnamely to compensate for the prismatic distorin this caseis primarily the prism 42. Since the tion. For most purposes this slitmay be 'consid- 50 pivot radius 'Il is greater than 1|) and since theered straight and to the degree that it approxi- Yangle between themotion of jaw 34 and the plane mates a straight line, it issubstantially perpentransverse to the light beam 'i3 is less thanthe"dicular to the plane of dispersion of the prism lcorresponding anglebetween the motionof' '36 v'42. `Either the entrance slit or the exitslit, or and plane transverse to '14, the effective change .both ofthem, may be curved. This curvature is 55 in width of the entrance slitis slightlygreater not an essential feature of. the present inventhanthat for the exit slit, but this slight differtion. The cylindricalcurvature of the mirror 4| ence is inconsequential. -is also apparentfromFig. 3. It will be noted Fig. 7 illustrates by curve Bil the energydisthat the'front surface 43 of the prism 4?. has tribution in theemergent light when the en- .longer radius of curvature than the rearsurface 50 trance and exit slits are effectively of equal width. :44.

The energy is a maximum at the center and falls Fig. 4 is similar toFig. 2 but shows some ci the oilp to zero at the edges of this band. Asimul- `m0unts and adjusting mechanisms for the optical 'taneousincrease in both the entrance and exit parts of the instrument. Theprism l2 is rigidly slit widths merely raises the curve to the positionattached to the support 50 by cross bar 5| and fl* 8|, increasingthetotal energy by the square of two screws 52. The term rigidlyattached inthe slit width and increasing the band width unicludes thoseelements for which manufacturing 'formly on both sides of the peak. Ifthe en- .adjustment is permitted. I n the case of the trance slit isheld at constant width and the exit prism 42 it is customary to providean abutment, 'slit is increased a curve such as 82 results. 1f the notshown, on the support 50 (with or without set 70 exit slit is heldconstant and the entrance slit screws or shims) for final criticalmanufacturing is increased in width, a similar curve 82 results.adjustment of the prism. As far asthe present Slight deviations frompurely triangular distri- 'invention is concerned this'prism is rigidlyatbution such as curve 8|! are inconsequential in tached to the support50. Alternatively fand in r most practical uses of monochrorriators.:`Also some respects more conveniently,th`e prism isA other aberrationstend to produce, suchdeviantions-ianywey. so-there. is no pointin'striving for absolute` equality of slit width. The wavelength scaleis here shown linearly; in. practice, a spectrum from 380 millimicronsto '100 millimicrons is so distributed on a circular arc that the midpoint of the arc is at 464 millimicrons. On a linear scale therefore,the triangle Sil is tipped slightly from the erect isosceles positionbut the distinction is extremely slight.

Fig. 4 and Fig. 8 show the mechanism for adjusting the movable planereflector 40. This reflector 40 is carried by a post 85 extendingvthrough an aperture 86 in the support 59. rihis host' 85 is rigidlyattached to a rigid member 81 whose ends are fastened to resilient metalrib. bons or springs 88 and 89. The ribbons are of thesame length andare attached at their other ends rigidly to the support 50 by the studs90. These spring members 88 and Sil are below the support 5c in Fig. 4.provided with an arm 9i extending toward the pla-newhieli is midwaybetween the ends of. the springs 88 and 89 and this arm 9| constitutes acam iol-lower substantially in that plane.. The

cam follower engages a rotary cam 92 rigidly v attached through a sleeve93 to the disc I9 which is rotated by the knob l5. The cam 92 may be oneither side of. the arm 9| and if a linear wave-length scale on the discis is desired the cam should be shaped accordingly, i. e. should besteeper at the blue end oi the spectrum. The cam shown is symbolic only;the exact shape is one-with radii directly proportional to the spec-Itral'variation in index of refraction of the prism used.

v This spring mounting of the movable mirror :40.provides accurate butsimple and convenient means for moving the mirror transversely Witt .outrotation. riheoretically even a single spring in either the positionshown at 88 or the position shown at 89 should provide motion oftranslation without rotation., but such a system would beouite sensitiveto error. The double spring construction is particularly rugged andstable .however- Figs. 9 and lo illustrate the principle oi themechanism just discussed. Ii the arrangement were: that shown in Fig.9,V wherein adjustment of.. the system is provided by pressure exerted.at'the point SS indicated by an arrow, the spring ,1

lwould tend to. remain straighter than the :springa and the member 81would be rotated slightly as shown. However, if the pressure is exertedat a point o1 as shown in Fig. 10 which point 91 is approximately in theplane 9,8. equidistant. from the ends of the springs a8 and 89, thetendency to rotate the member 81 is practically -elirriinatedx at leastfor all first and` second order factors. The distance. of a planeA suchaerea from a point such as 3S which is the. efective end of. theresilient ribbon 88, is of course measured perpendicularly as indicatedby the perpendicular |00. This arrangement is not absolutely criticaland the fact that the pressure point 91 may and usually does moveslightly out of the midway plane, does not introduce any appreciableerror. The pressure point need be only approximately in the plane inquestion.

.- The principle of operation of this specific em- .bodiment will now bedescribed with reference to Fig. l1. The focal circle ||l| with a radiusof' curvature of 2.58.1 mm. passes through the :exit slit 3l, through avirtual image |02 of the `ordnanceslit $2. as formed by the reector. 40.and 'through thepoint ||0` of the front .surface |04 The rigid memberB1 is i oi the virtual image |03 of prism 42 formed Aby the cylindricalmirror 4|. In practice the sui'.- face 43 of the prism. 42 mayconveniently be also on the circle |0l, but to indicate that the actualposition of the prism 42 is not critical, lt has been shown spaced fromthe circle |0|. A standard Fery monochromator would have an entranceslit |02, a prism at |03 and an exit slit at 31. The movable mirror 40and the cylindrical mirror 4| constitute features added in accordancewith the present invention.

The surface |04 has its center of curvature at the point |06 on thecircle |0| diametrically opposite thev prism position |03. The rearsurface |05 also has its center of curvature |01 on the circle l0|.

Radius of curvature of surface |04=Rr=511.4

Radius of curvature of surface |05=R2=469.1

The prism angle between surfaces |04 and |05 Effective thickness of theprism |03 at point 110=t==21 mm.

this is arbitrarily selected to give a convenient sZe prism.

The decision as to where to place the various components on the focalcircle |0| is not critical. In this particular case a prism at the pointilo would give minimum deviation to light of wavelength 464 mlllimicronscoming from the point |08 and returning to the point |08 afterreflection by the prism |03. To avoid physical interference of theparts, the' exit` slit 31 is arbitrarily placed at some distance alongthe circle |0| from the point |08. For light of wavelength 464 mu, theentrance slit must effectively be at a point |02, an equal distance onthe other side of point |08. It happens that the angle of incidence of aray trema point |08 to the point |0 on the surface IML-10x54 degrees 51minutes.

Movement of the mirror 40 parallel to itself moves the entrance slit|02. along a tangent to the circle |0|, which approximates motion alongthe circle itself. The actual entrance slit 32 ls also on this tangent.Motion of the jaws which constitute the slit so asto remain in properfocus has been fully described above in corinectlon with Figs'. 4, 5 and6'. Either the cntrance slit' or the exit` slit may be the one on thecircle |01 andthe other-slit will then have a. virtual image on thecircle |01. In either case motion of the movable mirror causes thespectrum, which lies along the circle |0| to move along this circle andtransversely to the exit slit.

The cylindrical mirror |09 is curved with its axis lying on the plane ofFig. 11. Any reasonable concave curvature reduces the astigmatismintroduced by the obliquity at which the rays strike the prism 42. Formaximum reduction of astigmatism this surface should have a radius ofcurvature equal to Where U=the distance |08 to |09 ofthemlrror 4| fromthe point |08., B=the angle of incidence of the'ray from |08 on themirror 4| and 4:R1 sin In tan (Io-A) as denedabove :511.4 sin 54 5'1'tan 24 51 in the example gillen 1971 mill.l

tion on the support with Also, in the above example U=l85.3 mm., B=71/2and the cylindrical radius of curvature of the mirror 4|'=7l5.7 mm.; anymirror with approximately this radius is satisfactory.

Having thus described the preferred embodiment of my invention, I wishto point out that it is not limited to the specific structure shown andis of the scope of the appended claims.

Iclaim:

l. A monochromator of the reflecting dispersing prism type, comprising asupport, a spherically curved reiiecting dispersing prism rigidlymounted on the support for receiving light from a line substantiallyperpendicular to the plane of dispersion and for forming a focusedspectrum intersecting said plane substantially along the arc of a circlewhich passes through said line, a concave cylindrical mirror with itscylindrical axis parallel to said plane rigidly mounted on the supportand oriented to reflect light from said line to the prism and to reflectspectrally dispersed light from the prism to said spectrum, entrance andexit slit defining means of adjustable slit width mounted in fixed posi-`perpendicular to said plane with both slits effectively on said circleat least one of them byhaving a virtual image thereof on said circle,the entrance slit being effectively at said line and the 'exit slitbeing effectively at a. point on said arc, 'a plane reflector forforming on said circle said virtual image of one of the slits, means onsaid support for moving the plane reflector so as to move said virtualimage substantially along said circle and means for illuminating andsending light through said 'entrance slit.

2. A monochromator according to claim l in which the plane reflectormoving means includes two spaced parallel resilient metal ribbons oi.equal length each rigidly attached at one end to said support, a rigidmember carrying the plane reiiector and attached to the other ends ofboth ribbons, a earn follower carried -by said rigid memberapproximately at a point in the plane which is equidistant from the endsof the ribbons and a rotary cam rotatably mounted on said support andengaging said cam follower.

3. A monochromator according to claim i, in which each slit definingmeans has a fixed jaw rigidly mounted on the support and a movable jawmovable to and from the fixed jaw for adjusting the slit width and whichincludes a member rigidly carrying both movable jaws and means forrotating said member through a small angle about a pivot to adjust bothslit widths simultaneously' and by approximating equal amounts relativeto the effective width of the light beams through the slits.

4. An optical system for a monochromator comprising entrance slitdefining means, exit slit defining means, a spherically curvedreiiecting dispersing prism and a concave cylindrical mirror all iixedlymounted relative to one another and a plane reflector for forming avirtual image of one of the slits, the plane reflector and the slits allbeing substantially perpendicular to the plane or" dispersion of theprism, the cylindrical axis of the mirror being parallel to said planeto reduce astigmatism introduced by the prism, said virtual image, thecylindrical mirror, the prism, the cylindrical mirror again and theother slit itself all being optically aligned in the order named, saidvirtual image, said other slit and the virtual image of the prism in thecylindrical mirror all being substantially on lthe circle which includesthe are on which the the slits substantially -prism forms a, focusedspectrum of aline also on said circle, the plane reflector beingoriented approximately radially to said circley and means for movingsaid plane reflector substantially paral'iei to itself to move saidvirtual image of one of the slits substantially along said circle,

5.'A monochromator of the reiiecting, spectral-dispersing-element typecomprising a support, entrance and exit slit defining means iixedlymounted on said support and means including a reflectingspectral-dispersing element rigidly mounted on the support, forreceiving light from the entrance slit and for focusing a spectrum alonga locus at and transverse to the exit slit, the slits beingsubstantially perpendicular to the plane of dispersion of the elementand a movable piane refiector also substantially perpendicular to saidplane of dispersion optically aligned to .reiiect the light once betweenthe element and one only of the slits, means on said support for movingthe plane reflector parallel to itself to move said spectrumtransversely across the exit slit and means for illuminating and sendinglight through said entrance slit. 6. A monochromator according to claim5 in which the plane rei'iector moving means includes two spacedparallel resilient metal ribbons of edual length each rigidly attachedatone end to said support, a rigid member carrying the plane reflectorand attached to the other ends of both ribbons, a cam follower carriedby said rigid member approximately at ai point in the plane which iseduidistant from the ends of the ribbons and a rotary cam rotatablymounted on said support and engaging said cam follower.

7. A monochromator` according to claim 5 in hich each slit definingmeans has a fixed jaw rigidly mounted on the support and a movable jawmovable to and from the fixed jaw'for adliusting the slit width andwhich includes a member rigidly carrying both movable jaws and means forrotating said member through a small angle about a pivot to adjust bothslit widths simultaneously and by approximately equal amounts relativeto the effective width of the light beams through the slits.

8. A monochromator according to claim 5 in which the spectrum focusingmeans comprises as the reiiecting spectral-dispersing element aspherically curved reliecti'ng element and includes a concavecylindrical mirror with its cylindrical axis parallel to said planerigidly mounted on said support aligned both to reflect light from theentrance slit to the element and to reflect spectrally dispersed lightfrom the element to said locus, the movable plane reflector beingoptically aligned between one of the slits and the concave cylindricalmirror.

9. An optical system for a monochromator comprising entrance and exitslit defining means and means including a plane reflector and areiiecting spectral-dispersing element for receiving light from theentrance slit for reiiecting it once only at said plane reflector andfor focusing' a spectrum along a locus at and transverse to the exitslit, the slit defining means and said element all being fixedly mountedrelative to one another with the slits substantially perpendicular tothe plane of dispersion of the element, means for supporting the planereiiector also perpendicular across the exit slit.

10. An optical system according to claim 9 in which the spectrumfocusing means comprises as the reflecting spectral-dispersing element avpersed light into focus as Vmounted on said support,

.the exit slit, the

defining means and a concave cylindrical mirror on the same support withall rigidly mounted the cylindrical axis of the mirror parallel totheplane of dispersion of the element and oriented to receive lighteffectively from the entrance slit and to reflect it to the element andalso to receive this light again after reflection and dispersion by theelement and to reflect the disn a spectrum effectively at the exit slit.

12. An optical system according to claim 1l in which said element is aspherically curved reiiecting prism. l

13. A monochromator comprising a support, entrance and exitslit-defining means xedly an optical system including aspectral-dispersing element for receiving light from the entrance slitand for focusing a spectrum along a locus at and transverse to slitsbeing substantially perpendicular to the plane of dispersion of theelement and a movable perpendicular .cally aligned the element and tosaid planev of dispersion optito reect the light once between one onlyofthe slits, a mount plane reflector also substantially- 1'2 for movingthe plane reector parallel to .itself to move said spectrum transverselyacross the exit slit, said mount consisting of two spaced parallelresilient metal ribbons of equal length each rigidly attached at one endto said support,

.a rigid member carrying the plane reiiector and attached to the otherends of both ribbons, a cam follower carried by said rigid member approximately at a point in the plane which is equidistant from the endsof the ribbons, a rotary cam rotatably mounted on said support andengagingsaid cam follower, and means for illuminating and sending lightthrough said entrance slit.

KENNETH G. MACLEIBH..

. 'References Cited in the file of this patent UNITED STATES PATENTSNumber Name Date 617,684 Friez v. Jan. l0, 1899 1,281,926 Fecker Oct.15, 1918 1,399,743 Brody 1 Dec. 13, 1921 1,711,768 Bausch et al May 7,1929 1,711,909 Stalcup May 7, 1929 2,021,232 Fassin Nov. 19, 19352,305,778 Hansen Dec. 22, 1942 .2,408,512 Gradisar Oct- 1, 19462,453,164 Swings Nov. 9, 1948 2,499,322 Mather et al. Feb. 28, 19502,504,949 Fouiounis Apr. 25, 1950 2,548,548 Menzies et al. ...1--- Apr.10, 1951 2,587,451 Farrand n n--..s n... Feb. 26, 1952 2,594,334 MillerApr.. 29, 1.952

FOREIGN PATENTS Number Country Date 550,932 Great Britain 1-- Feb. 1,1943

